[ubuntu] Need disk partition space advice

For various reasons it's a good idea to install Ubuntu on 2 partitions: / (system) and /home (data) - Drive letters are only used on Windows, we don't have them over here, we just use paths for everything.

20G is more than enough for the Ubuntu system, and the rest you can leave for your Win system. You can then put your Ubuntu /home partition on your separate data drive. To achieve this, the best thing to do is run a Live DVD and fire up Gparted. Create a 20G ext4 partition on your system drive and a swap partition to match the amount of RAM you have, and another ext4 for your /home on the other drive. Then during install when you're prompted where to install select 'something else' and set the 20G to / and the other to /home and also the swap. Note: make sure you select the system drive for GRUB (on the same screen). GRUB overwrites the Windows MBR so it's a good idea to have a recovery CD handy just in case anything goes wrong and you need to reinstall MBR.

Your Windows won't slow down in any way because when you dual boot you are not sharing any resources at all with the other OS.

Your motherboard probably has 2 boot modes, UEFI and BIOS. Make sure you use the same method for both or you could run into problems.